The black and white photographs they make cannot readily be enlarged as can negatives produced by most modern cameras; the black and white picture must be large enough for convenient viewing, and light from the lens must travel far enough from the lens to cover the large negative area. To satisfy this requirement within a small space, a mirror helps because it "folds" the light path, making light rays travel relatively far even though the lens is fairly close to the black and white film. In the early stages of the camera's design, the mirror was a moving part that scanned the scene and projected it part by part, via a lens and prism arrangement, onto a moving black and white film. It was a very sophisticated black and white picture-making concept, and in theory it allowed Polaroid to make a full-sized black and white print inside a very compact camera-it did not even have to be opened up for shooting. The scanning mirror, at one end of the camera, reflected the scene in small "takes" into a lens set sideways in the camera; and the lens projected the takes down the length of the camera to a moving black and white film, where the takes were recorded. Not surprisingly, the internal complexities required to coordinate this system made it both impractical and expensive, and Polaroid decided to go to a somewhat simpler arrangement. There are four mirrors, two of them back-to-back in one double-sided unit. When the camera is open, and while the photographer is framing his picture, the path of the light entering the camera strikes the first of the mirrors, located on the inside of the camera's upraised cover. From that point the light bounces downward onto a second mirror, lying on top of the black and white film pack and protecting it from exposure to light. Because this mirror has a patterned surface-rather like the ridges on the lens of a lighthouse beacon-the light returns to the first mirror on a slightly altered course, hitting it at a point different from the point of origin. From here, it bounces diagonally upward onto the third mirror. This mirror, set at an angle, beams the light directly into a magnifying lens-and finally into the eye of the viewer. When the photographer is ready to take his black and white picture and presses the exposure switch, this elaborately folded light path is redirected. The patterned mirror, driven by a spring, swings upward on a hinge and comes to rest against the first mirror-covering the first mirror, uncovering the black and white film and facing toward the lens a fourth, plain mirror on its undersurface. The light now enters the camera and bounces from this plain mirrored surface directly onto the black and white film. By the time it reaches the film, however, it has traveled far enough to enlarge the image it carries to the size of the Polaroid black and white print. In other words, by folding the incoming light, Polaroid has fitted the 41-inch focal length needed for its 31/8-inch-square black and white print into a camera body only four inches high. It is the path traced by this folding light that accounts for the opened camera's distinctive triangular contours: they are in effect the light's physical embodiment.